1. Field of Invention
The present invention relates to radiation detection devices and, in particular, to an extended range thermal displacement-based radiation detecting device.
2. Description of the Background Art
Conventional radiation detection devices, such as thermal cameras, may operate using a thermo-electric mechanism to detect radiation signals. For example, an infrared signal distribution may be directed onto a semiconductor imaging device and current or voltage changes may be measured to indicate a temperature rise in each pixel. However, these technologies have major deficiencies. In the present state of the art, the sensitivity of an infrared camera may be fundamentally limited by electron thermal noise.
Cooling may be used to mitigate this limitation, but this additional complexity results in a bulky radiation detection device with high power consumption. Moreover, the fabrication of these devices is intrinsically hampered by the need for an electrical interconnect to each pixel as well as integrated scanning readout electronics. All of these factors have kept the cost prohibitively high for many commercial applications, as well as making the detectors expensive to produce and difficult to scale in array size (i.e., resolution) due to low fabrication yield of the detection sensor.
A new type of thermal detectors based on micro-electrical-mechanical systems (MEMS) has emerged in recent years. This type of IR detectors, in which micromechanical photon detector structures are fabricated by a few steps using conventional materials and processes, have opened an unprecedented opportunity for a low cost state-of-the-art IR imager. The readout can be a capacitive readout such as disclosed in U.S. Pat. No. 5,623,147 issued to Baert et al., or a piezoresistive readout such as disclosed in U.S. Pat. No. 6,118,124 issued to Thundat et al., or an emissive readout such as disclosed in U.S. Pat. No. 6,140,646 issued to Busta et al. However, the requirement of providing an electrical readout contact to the sensor element may reduce sensitivity as a result of thermal signal leak.
A different approach to uncooled thermal imagers based on passive thermal bending and optical readout has been reported. The use of bimaterial microcantilevers for temperature and radiation sensing has been disclosed in Baert et al. '147 and demonstrated by several groups, including Oden 1996, Perazzo 1999, Ishizuya 2002, and Manalis et al 1997. In particular, Manalis fabricated the arrays of bimaterial microcantilevers for infrared imaging and demonstrated direct conversion from an infrared to a visible signal.
This type of approaches utilizes a passive sensor without requiring electrical contacts. One advantage is that electron thermal noise may be reduced and sensor fabrication may be simplified without the need for pixel-level driving circuitry integration. However, early designs, such as disclosed in Thundat et al. '124 and in U.S. Pat. No. 5,929,440 issued to Fisher, use bimaterial cantilevers directly as the sensors are intrinsically and significantly poor in sensitivity; rendering them impractical for commercial applications. In these imager designs, bimaterial cantilevers are exposed to thermal radiation that heats up the cantilevers, leading in turn to bending as a result of the thermal expansion mismatch of the two cantilever materials. The bending can be detected by an optical beam, thus providing a signal proportional to the thermal radiation intensity. However, early designs have had poor radiation absorption efficiency and large sensor mass that lead to inefficient thermal bending devices. This design may also leave the sensor component directly exposed to environmental temperature fluctuation and hence cannot be used as “uncooled.”
One type of optical thermal bending imager design has been disclosed by Tohru Ishizuya et al. in U.S. Pat. No. 6,080,988. In this design, the sensing element and the bending portion are separated for optimization. Each sensor pixel is made of an optical absorbing cavity that is supported by two bimaterial cantilevers. The cavity receives infrared radiation and converts infrared rays into heat that conducts into the connecting cantilever arms, resulting in bending respect to the substrate base due to bimetal principle.
In this approach, the radiation is first efficiently converted into heat by the optical cavity sensor, and as the heat conduct into the supporting bimaterial arms, the cantilever arms are then bent corresponding to the heat and thus deflect the readout light beam accordingly. This design achieves very high sensitivity by separating the absorption and displacement with a high absorption efficient yet lightweight optical cavity and narrow bimaterial cantilever bending strip arms. However, the design lacks many features that are important for practical applications.
For example, the uncooled IR imagers demonstrated to date all require one to two order of magnitude improvements in sensitivity in order to displace the established cryogenically cooled semiconductor IR imagers. No semiconductor materials known today can provide such sensitivity improvement without sacrificing responses time. Having a low thermal sensitivity means that a larger pixel size is required so as to provide adequate thermal absorption. Low thermal sensitivity has also limited the achievable dynamic range in the current state of the art.
What is needed is a thermal displacement radiation detector for radiation detection that provides extremely high IR sensitivity and has low power requirements.